The post-viral fatigue syndrome occurs sporadically and in local outbreaks (Los Angeles, Akureyri, Royal Free Hospital). The clinical picture is marked by long-lasting muscular fatigue directly following an acute infection. Other conditions associated with pronounced fatigue must be excluded. The diagnostic criteria set up by Centers for Disease Control (CDC) are the ones most commonly used. Aetiology and pathogenesis are unknown. Coxsackie B-virus seems to be associated with some cases at least. Immunological and endocrinological aberration, morphological changes in mitochondria and reduced cerebral blood perfusion have been demonstrated in some patients. There is no specific therapy. It is important for the patient that the symptoms be accepted by the doctor and society in general.